1. Field of the Invention
The present invention relates to fluid level monitors for monitoring the level of fluid in a vessel.
2. Background Information
A known method of monitoring fluid levels in a vessel is to perform a time of flight measurement. A transducer emits a pulse of ultrasound from either above or below a fluid level and the reflection of the ultrasound from the fluid surface is detected by the transducer and the time taken for the ultrasound to travel from the transducer to the fluid surface and back is measured. Using the knowledge of the velocity of the ultrasound in the fluid, or acoustic conductor, it is then possible to calculate the fluid level.
A further known method of monitoring fluid levels in a vessel is to position a pair of transducers, one vertically above the other on the vessel wall. One of the transducers emits a pulse of ultrasound from either above or below the fluid level, and the other transducer detects the ultrasound travelling vertically in the vessel wall. The ultrasound propagating in the vessel wall is damped by any fluid contacting the vessel wall. The amplitude of the detected ultrasound is a measure of the level of the fluid in the vessel. This method, however, is not completely satisfactory because the amplitude of the detected ultrasound is dependent also upon the transducer sensitivity as well as the fluid level. This method would require careful calibration to compensate for the transducer sensitivities.
The present invention seeks to provide a novel apparatus for monitoring the level of fluid in a vessel.
Accordingly, the present invention provides a fluid level monitor for monitoring the level of fluid in a vessel comprising a propagation member which allows the propagation of stress waves therethrough, at least one acoustic emission transducer acoustically coupled to the propagation member, the at least one acoustic emission transducer being arranged to transmit one or more stress wave pulses into the propagation member, the stress wave pulse propagating in the propagation member for a period of time to produce a diffuse stress wave field in the propagation member, the diffuse stress wave field being damped by any fluid in contact with the propagation member, the at least one acoustic emission transducer detecting the stress wave propagating in the propagation member and producing an electrical signal corresponding to the level of diffuse wave field, analyzer means to analyse the electrical signal to measure the damping of the diffuse stress wave field, the amount of damping of the diffuse stress wave field being indicative of the level of fluid in the vessel contacting the propagation member.
The propagation member may be formed integral with the wall of the vessel.
At least a first portion of the propagation member may extend into and be positioned within the vessel.
A second portion of the propagation member may be positioned outside of the vessel, the at least one transducer is positioned on the second portion of the propagation member.
A first transducer may be arranged to transmit stress wave pulses into the propagation member, a second transducer is arranged to detect the stress waves propagating in the propagation member.
The analyzer means may measure the damping of the diffuse stress wave field by measuring the decay rate of the diffuse stress wave field.
The decay rate may be measured using the slope of the envelope of the diffuse stress wave field.
The decay rate may be measured for a transient stress wave pulse.
The decay rate may be measured for repeated stress wave pulses.